Method of drilling in permeable formations



United States Patent 3,223,185 METHOD OF DRILLING IN PERMEABLE FORMATIONS Lodewijk Johannes Schmit Jongbloed and Nicolaas Honk Van Lingen, Rijswijk, Netherlands, assignors to Shell Oil Company, New York, N.Y., a corporation of Delaware No Drawing. Filed Dec. 7, 1961, Ser. No. 157,814 Claims priority, application Netherlands, Jan. 18, 1961, 260,171 11 Claims. (1. 175-65) This invention relates to the drilling of oil and gas wells and pertains more particularly to well drilling operations wherein a drilling fluid is circulated in a well to temporarily seal the well wall at a point below the surface of the formation being drilled and not on the surface thereof.

When drilling deep holes, such as oil and gas wells, in permeable formations with the use of a drilling fluid pumped down a drill string or pipe string and out a drill bit to be circulated up the well to the top thereof, it is the practice to incorporate solid particles in the drilling fluid. When the drilling fluid penetrates into the formation owing to the pressure prevailing in the hole caused by the hydrostatic head of the column of fluid, these particles deposit on the wall of the hole (including the bottom), thus forming a filter layer which hinders, and eventually substantially stops further egress of liquid from the hole to the formation. The filter layer thus forms a separation between the high liquid pressure zone in the hole and the lower liquid pressure zone in the liquidsaturated formation. Thus, at the wall of the hole there is a high pressure drop over the filter layer formed.

It has now been found that this pressure drop has a very unfavorable effect on drilling progress, which, for instance, may be expressed in the increase in drilling hole depth obtained per revolution of the drilling bit. This unfavorable effect is disclosed, among other things, by the fact that, when drilling at shallow depth, drilling progress is usually better than when drilling at a great depth under otherwise similar conditions as regards, for instance, the nature of the formation, bit load, and the speed at which the bit rotates, since when the hole increases in depth there is usually also an increase in pressure difference between the drlling fluid and the liquid in the formation, and consequently an increase in the said pressure drop. This phenomenon may be explained by the fact that when there is a high pressure drop over the filter layer the bit force required to loosen a particle or cutting from the drilling hole bottom is considerable owing to the difference in liquid pressures acting on this particle on the side of the drilling hold and the side of the formation; this pressure difference keeps the particle pressured close against the formation or held against the bottom of the hole, and hinders the separation of this particle from the rest of the formation by the rotating drill bit.

It is a primary object of the present invention to provide a method of drilling wells in a manner so as to reduce the pressure differential of fluid at the face or wall of a hole or well in contact with a drill bit.

A further object of the present invention is to reduce and preferably substantially equalize the pressure exerted on a formation being drilled by a circulating column of drilling fluid in the well, the pressure reduction or equalization taking place in the formation to a depth about the size of bit cuttings.

Another object of the present invention is to provide a method of drilling wells wherein a temporary substantially impermeable barrier is formed below the formation surface.

With the use, according to the invention, of a drilling 3,223,185 Patented Dec. 14, 1965 fluid which, though temporarily and substantially sealing the formation, does not cause a high pressure drop at the wall or face of the drilling hole, this phenomenon or holding down of bit cuttings is counteracted or eliminated, thereby ensuring better drilling progress. According to the invention, the formation is not or substantially not sealed at the boundary between the formation and the hole, viz., at the wallet the hole, but as much as possible in the formation itself at some distance from the said wall. The pressure prevailing in the formation near the wall of the hole then becomes the same or substantially the same as in the hole. Consequently, a particle loosened by the bit from the wall or bottom of the hole is exposed to substantially the same pressure from all sides, so that no additional force is required by a pressure difference to break the connection of the particle with the solid formation.

In order to ensure that, with the aid of the drilling fluid, the barrier separating the zone of high drilling fluid pressure from the zone of lower formation-liquid pressure (pore pressure) is at some distance from the wall of the hole, the drilling fluid should penetrate into the formation without forming an impermeable layer on the wall of the formation, but that the pores are not clogged until the liquid has permeated into the formation, thus preventing further penetration of the liquid into the formation. For best drilling performance, the sealing barrier should be formed in the formation a distance from the face of the well borehole at least equal to about the average bit cutting size.

This can be achieved by a suitable choice of the type and composition of the drilling fluid taking into account the type and contents of the formation.

It is obviously essential that the drilling fluid should not form a plastering layer on the drilling hole wall as is the case with the conventional drilling fluids. When the aqueous constituents of presently known drilling fluids penetrate into the formation, most of the solid particles of different sizes are left behind on the wall of the drilling hole where they soon build up a filter layer which is impermeable or substantially impermeable to liquid. Hence, the drilling used according to the invention should not contain an amount of solid particles of the kind capable of forming a substantially impermeable filter layer, but the presence of non-sealing solid particle-s or cuttings may be permitted.

There are several ways in which the pores in a barrier may be formed or a formation may be clogged with drilling fluid. It is possible, for instance, to make use of the fact that in some formations the pores are filled with a liquid containing dissolved salts, especially sodium chloride, and to cause the drilling fluid to contain a dissolved substance which, together with the salt in the formation liquid, produces a precipitate or flocculation which narrows or even completely seals off the passage in the pores a distance away from the face of the hole. Suitable substances for dissolving in the drilling fluid are lead salts, such as lead acetate, and sodium soaps (e.g., sodium stearate). When a drilling fluid of either type penetrates into the formation pores filled with a salt solution, the two liquids come into contact as a result of diffusion and mixing to form a precipitate, but the precipitate is not formed until the drilling fluid has penetrated the formation to a certain depth.

The above-described effect was found from a number of experiments in which various liquids were forced under a pressure of 25 atm. g. through a permeable stone, viz., Gildenhauser sandstone with a permeability of 3 darcies which was saturated with a salt solution of 10 grams of sodium chloride per cc. of water. When clean water was used for this purpose, a constant quantity of cc.

per minute continued to flow through the stone. But when an aqueous solution of lead acetate was used containing 45 grams of lead acetate per 100 cc. of water, the quantity of liquid that passed through (loss of filtrate) after two minutes was only 45 cc. per minute. The area where the precipitate had formed was found to be about 5 cm. distant from the contact area or area of penetration of the lead acetate into the stone. The effect was still more pronounced when a soap solution was used containing 2 grams of sodium soap, i.e., sodium stearate, per 100 cc. of water; after 2 minutes the loss of filtrate was only 8 cc. per minute, the barrier formed by the precipitate being 3 cm. distance from the area of entry.

When drilling through salt layers, a drilling fluid is generally used which has a high salt content in order to prevent leaching out of the salt layers. In such a case the above-mentioned substances are obviously practically useless. In order to obtain a precipitate in the pores of a formation, use may also be made of a substantially nonaqueous drilling fluid, which forms a precipitate when diluted with salt or fresh formation water. A suitable drilling fluid, for example, is one which consists of a solution of bitumen in pyridine, or of paratfin wax in methyl ethyl ketone. In an experiment with the latter system, carried out under the same conditions as specified above for the experiment with sodium soap, the quantity of filtrate Was found to be 10 cc. per minute after 120 seconds.

Methods other than the one by which a precipitate is formed may be used for clogging formation pores. In one method use is made of a great increase in viscosity of the drilling fluid after it has penetrated into the formation. A possible example is an aluminum carboxymethyl cellulose, dissolved in an alkaline liquid, which solution shows a great increase in viscosity with such decreasing alkalinity as results from dilution with formation water. It is essential that the cellulose material be of the type that will dissolve completely and go into the formation and not one that forms a sheath or filter cake on the wall of the well borehole.

In this connection it may be remarked that the presence of solid particles of great size in the drilling fluid does not give rise to the formation of a plastering layer. If the particles are so large that the interstitial pores are of the same order of magnitude as those present in the formation, the flow pattern is not essentially changed by a layer of such solid particles on the wall of the formation. But the presence in the drilling fluid of solid particles with a complete range of dimensions is detrimental to the object envisaged by the invention since in this case the pores remaining between the larger particles in the layer deposited are filled with particles of smaller dimensions until a substantially impermeable layer is formed. Hence, the drilling fluid should not contain an amount of solid sealing or plastering particles which would form an impermeable sheath on the wall of the formation.

Consequently, if the invention is to be applied, in order to weight the drilling fluid to a specific gravity of 1.15 or higher, as is necessary in many cases, use cannot be made of solid particles suspended in the drilling fluid and having a largely continuous'range of dimensions. But these dimensions should be chosen with care or the fluid weighted with such substances as sugar, glycerine, or water-glass, dissolved in the drilling fluid. With the use of an aqueous solution of lead acetate without other additions, a specific gravity of 1.25 can be attained.

Experiments such as the following demonstrate the possibility of achieving the ultimate object of the invention, viz., increasing the drilling progress.

On a pilot drilling installation wherein drilling fluid was pumped down a drill or pipe string and discharged out a drill bit, a drilling progress of 0.08 mm. per revolution was obtained with the use of conventional drilling fluid (clay/water fluid) when drilling Gildenhauser sandstone saturated with a salt solution (10 grams of sodium chloride per 100 cc. of water), by means of a diamond bit with a diameter of 10 cm. under a bit load of 1,500 kg., the drilling hole pressure being 50 atm. g. and the pore pressure 0 atm. g., Whereas at a drilling hole pressure of 0 atm. g. and a pore pressure of 0 atm. g. progress was 1.1 mm. per revolution. The high drilling hole pressure therefore resulted in a reduction of over When drilling at the same high drilling pressure, under otherwise similar conditions but with the use of a 2% stearate soap solution, drilling progress was 0.8 mm. per revolution, i.e., 10 times the progress obtained with the use of clay fluid.

It should be noted that in cases in which solid particles or cuttings which might form a plastering layer are loosened from the formation by the drilling operation itself, it is important that these particles immediately after they are formed are removed as quickly as possible from the region where the bit attacks the formation (by creating in situ a flow of drilling fluid of high velocity), and

that these particles are removed from the circulating drilling fluid by screening, filtering, settling or centrifuging, before the latter is pumped back into the drilling hole.

Finally, the following should be noted. When drilling a well, permeable formations will not usually be drilled over the entire depth; impermeable and very slightly permeable formations will also be encountered. If, however, permeable formations are anticipated over the depth to be drilled through, a drilling fluid according to the invention will usually be employed for the entire drilling operation. Although the effect described for permeable formations will not occur when drilling through impermeable or only very slightly impermeable formations, the absence in the drilling fluid of solid particles which can form a dense layer or compact mass may also be of advantage in this case.

In the method of the present invention the zone of pressure difference, normally at the face or wall of the well, is maintained at a point substantially at or below the depth of the chips or cuttings to reduce the hold-down force normally exerted on the bottom hole formation by the column of fluid in the well. During the drilling of a well in accordance with the present method, the pressure drop front is continually moved below the area in which a bit is forming chips or cuttings.

After drilling operations in a well have been completed, the substantially impermeable barrier just below or into the face of the borehole wall may be removed at least in that portion of the borehole traversing the producing formation of the well. The previously induced barrier in the formation may be removed by any one of several methods depending upon the type of barrier employed and the characteristics of the formation. Many barriers are readily removed by back-flushing liquid from the producing zone or putting the well on production so that oil or gas from the formation forces the barrier back into the well. Alternatively, a barrier, depending on the type used, may be dissolved or destroyed by an acid or a solvent, either organic or inorganic, depending on the material to be attacked. In other cases viscosity reducing agents may be employed.

We claim as our invention:

1. A method of drilling wells formed by a drill bit at the lower end of a drill string while employing a drilling fluid, said method comprises (a) rotating said drill string with the drill bit bearing against the face of the formation being drilled,

(b) pumping a drilling fluid down the drill string,

(c) discharging the drilling fluid through the bit at the bottom of the rotating drill string and against the bottom of the well,

((1) circulating the drilling fluid up the well outside the rotating drill string at a rate sufficient to maintain the face of permeable formations traversed by the bit in a fluid-permeable condition, and

.(e) substantially reducing the permeability of said formation at a point in the formation at least substantially at chip depth, thereby maintaining a pressuredrop at said point to reduce the hold-down force of the circulating column of fluid exerted against chips being formed by the bit.

2. A method as claimed in claim 1, characterized in that the drilling fluid contains solid particles of such dimensions that they do not form an impermeable layer on the wall.

3. A method as claimed in claim 1, characterized in that the specific gravity of the drilling fluid is increased by the presence of substances which are soluble therein.

4. The method of claim 1 wherein said drilling fluid contains a dissolved substance and including the step of preciiptating at least a portion of said dissolved substance within the formation at a point displaced from the face thereof by contacting said dissolved substance with the liquid of the formation.

5. A method as claimed in claim 4, characterized in that the drilling fluid consists of a solution of bitumen in pyridine.

6. A method as claimed in claim 4, characterized in that the drilling fluid consists of a solution of paraffin wax in a solvent such as methyl ethyl ketone.

7. The method of claim 1 including the step of increasing the viscosity of said drilling fluid within the formation at a point displaced from the face thereof by contacting said drilling fluid with formation liquid.

8. A method as claimed in claim 7, characterized in that the alkaline drilling fluid contains aluminum carboxymethyl cellulose in solution.

9. The method of claim 1 including the step of subsequently removing the substantially impermeable barrier within the formation adjacent a producing formation traversed by said drill bit.

10. A method of drilling wells formed by a drill bit at the lower end of a drill string while employing a drilling fluid, said method comprises (a) rotating said drill string with the drill bit bearing against the face of the formation being drilled,

(b) pumping a driling fluid down the drill string,

(c) discharging the drilling fluid through the bit at the bottom of the rotating drill string and against the bottom of the well,

(d) circulating the drilling fluid up the Well outside the rotating drill string at a rate suflicient to maintain the face of permeable formations traversed by the bit in a fluid-permeable condition,

(e) substantially reducing the permeability of said formation at a point in the formation at least substantially at chip depth, thereby maintaining a pressuredrop at said point to reduce the hold-down force of the circulating column of fluid exerted against chips being formed by the bit,

(f) continually moving the pressure drop front during drilling operations below the area in which chips are being formed by the drill bit.

11. A method of drilling wells formed by a drill bit at the lower end of a drill string while employing a drilling fluid, said method comprises (a) rotating said drill string with the drill bit bearing against the faces of the formation being drilled,

(b) pumping a drilling fluid down the drill string,

(c) discharging the drilling fluid through the bit at the bottom of the rotating drill string and against the bottom of the well,

(d) circulating the drilling fluid up the Well outside the rotating drill string at a rate suflicient to maintain the face of permeable formations traversed by the bit in a fluid-permeable condition,

(e) substantially reducing the permeability of said formation at a point in the formation at least substantially at chip depth, thereby maintaining a pressuredrop at said point to reduce the hold-down force of the circulating column of fluid exerted against chips formed by the drill bit,

(f) continually moving the pressure drop front during drilling operations below the area in which chips are being formed by the drill bit, and

(g) removing from the drilling fluid at the top of the well formation-sealing chips and materials of a size and nature and quantity which form sealing.

References Cited by the Examiner UNITED STATES PATENTS 1,421,706 7/1922 Mills 166-38 2,156,219 4/1939 Dunn 166-29 2,801,218 7/1957 Menaul 166-42 2,801,699 8/1957 Sayer et al. 166-29 2,867,278 1/1959 Mallory et al. 166-33 2,896,716 7/ 1959 Spurlock 166-38 3,040,821 6/1962 Widess -70 BENJAMIN HERSH, Primary Examiner.

BENJAMIN BENDETT, Examiner. 

1. A METHOD OF DRILLING WELLS FORMED BY A DRILL BIT AT THE LOWER END OF A DRILL STRING WHILE EMPLOYING A DRILLING FLUID, SAID METHOD COMPRISES (A) ROTATING SAID DRILL STRING WITH THE DRILL BIT BEARING AGAINST THE FACE OF THE FORMATION BEING DRILLED, (B) PUMPING A DRILLING FLUID DOWN THE DRILL STRING, (C) DISCHARGING THE DRILLING FLUID THROUGH THE BIT AT THE BOTTOM OF THE ROTATING DRILL STRING AND AGAINST THE BOTTOM OF THE WELL, (D) CIRCULATING THE DRILLING FLUID UP THE WELL OUTSIDE THE ROTATING DRILL STRING AT A RATE SUFFICIENT TO MAINTAIN THE FACE OF PERMEABLE FORMATIONS TRAVERSED BY THE BIT IN A FLUID-PERMEABLE CONDITION, AND (E) SUBSTANTIALLY REDUCING THE PERMEABILITY OF SAID FORMATION AT A POINT IN THE FORMATION AT LEAST SUBSTANTIALLY AT CHIP DEPTH, THEREBY MAINTAINING A PRESSUREDROP AT SAID POINT TO REDUCE THE HOLD-DOWN FORCE OF THE CIRCULATING COLUMN OF FLUID EXERTED AGAINST CHIPS BEING FORMED BY THE BIT. 